Yes, it can be swapped back using the same tools that were used to put it on Cricket in the first place.

I wouldn't suggest doing this yourself. There are probably shops in your area that specialize in flashing phones.

You can register the ESN to Sprint to get basic service, but the data portion of your phone's software has been modified to make it work with Cricket.

Alternatively, you could probably load a custom Sprint ROM from sdx-developers on your phone, that should restore all the data settings to Sprint, then you'd just need to register the ESN on Sprint's site.